It's all smut

Corn smut Plant diseases are an important constraint on worldwide crop production, accounting for losses of 10–30% of the global harvest each year and represent a significant threat to global food security. Fungal pathogens can broadly be divided into two groups – the biotrophs and necrotrophs. Biotrophic pathogens are parasites that have evolved the means to grow within living plant cells without stimulating plant defence mechanisms. This means that they are able to spread rapidly throughout plant tissue while, at the same time, diverting nutrients from the living plant to fuel their own growth at the expense of plant productivity. In contrast, the necrotrophic pathogens use toxins and depolymerising enzymes to kill and degrade plant cells, consuming the resulting products.

In order to grow, a plant pathogenic fungus must secure an organic carbon source from the plant. In most plant diseases, however, we have little idea of what constitutes the major carbon source for an invading fungus during growth in plant tissue. How do biotrophic plant pathogens acquire nutrients efficiently from a living plant cell? A study published in PLoS Biology provides a significant advance in understanding the mechanism by which a plant pathogenic fungus is able to acquire nutrients in plants. The conclusion is that sucrose, which constitutes the most abundant storage sugar within plants and the product of photosynthesis, is directly utilised by invading pathogens without the need for its extra-cellular degradation by fungal secreted invertases.

A Novel High-Affinity Sucrose Transporter Is Required for Virulence of the Plant Pathogen Ustilago maydis. 2010 PLoS Biol 8(2): e1000303 doi:10.1371/journal.pbio.1000303
The plant parasitic fungus Ustilago maydis is a biotrophic pathogen that depends on live plant tissue for development. It is highly adapted to maize (Zea mays), where it causes the corn smut disease. Fungal cells growing within the plant apoplast are surrounded by the host plasma membrane at all growth stages, thereby establishing tight interaction zones with the host cells that assure optimal access to host-derived nutrients, including organic carbon sources. Here, we focus on the previously unknown feeding mechanisms of this plant pathogen within its host plant. We identified a fungal plasma membrane transporter, Srt1, that is expressed exclusively after plant infection and that turns out to be essential for virulence development of Ustilago in infected plants. Srt1 is the first characterized fungal transporter that allows direct utilization of sucrose without extracellular hydrolysis into monosaccharides, the carbon form more commonly taken up by pathogenic fungi. It is highly specific for sucrose, and its affinity largely exceeds that of equivalent plant transporters. This not only provides advantages for the carbon acquisition by the pathogen, but quite likely also offers a mechanism to prevent induction of plant defense responses known to occur upon apoplastic sucrose hydrolysis.

Related:

  • Living the Sweet Life: How Does a Plant Pathogenic Fungus Acquire Sugar from Plants?

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